The present invention relates to ultrasound imaging and, more particularly, to detection of a level of a contrast agent, where the contrast agent is susceptible to destruction by an ultrasound signal.
Ultrasonic diagnostic imaging systems are capable of imaging and measuring the physiology within the body in a completely noninvasive manner. Ultrasonic waves are transmitted into the body from the surface of the skin and are reflected from tissue and cells within the body. The reflected echoes are received by an ultrasonic transducer and processed to produce an image or measurement of blood flow. Diagnosis is thereby possible with no intervention into the body of the patient.
Materials known as ultrasonic contrast agents can be introduced into the body to enhance ultrasonic diagnosis. Contrast agents are substances that strongly interact with ultrasonic waves, and return echoes that may be clearly distinguished from those returned by blood and tissue. One class of substance that has been found to be especially useful as an ultrasonic contrast agent is encapsulated gas, in the form of microbubbles.
Microbubbles present a significant acoustic impedance mismatch in the body, and a nonlinear behavior in certain acoustic fields that is readily detectable through special ultrasonic processing. Gases that have been stabilized in solutions in the form of tiny microbubbles are infused into the body and survive passage through the pulmonary system and circulate throughout the vascular system. Microbubble contrast agents are useful for imaging the body""s vascular system, for instance, as the contrast agent can be injected into the bloodstream and will pass through the veins and arteries of the body with the blood supply until filtered from the blood stream in the lungs, kidneys and liver.
A coated microbubble is a microbubble contrast agent that is covered with a thin biodegradable coating or shell. Coated microbubbles are suspended in an aqueous solution for infusion into the blood stream. Coated microbubbles have the advantage of being stable in the body for a significant period of time, as the shells serve to protect the gases of the microbubbles from diffusion into the bloodstream. A size for the microbubbles is chosen to enable the microbubbles to pass through capillary beds.
At moderately high sound pressure amplitudes the acoustic pressure waves can cause the shells of coated microbubbles to rupture, freeing the bubbles to behave as non-coated microbubbles until they diffuse into the bloodstream. Acoustic energy can induce a microbubble to move in a nonlinear manner, itself a detectable ultrasonic phenomenon.
In some applications, a user of an ultrasound system may wish to distinguish a region into which blood flows rapidly from a region into which blood perfuses. Blood flow, as compared to perfusion, is a relatively fast, higher rate of blood transfer, such as that found in a heart ventricle, an artery or a vein. On the other hand, perfusion is a relatively slow diffusion of blood through a network of capillaries in an organ, such as the myocardium.
Ultrasound imaging and measurement of blood flow is relatively easy as compared to imaging and measurement of perfusion. Ultrasound echoes from higher flow rates can be separated from tissue echoes, which have greater magnitudes, by evaluating signal components that change phase quickly between successive measurements at a given location. For example, flowing blood can be distinguished from stationary tissue on a basis of a Doppler frequency shift of an echo signal.
The Doppler technique is not effective for perfusion imaging because an organ through which the blood is perfusing often moves substantially as compared to the relatively slow movement of the perfusing blood. Consequently, the blood cannot be distinguished from the organ. This situation is particularly relevant in a case where the heart muscle is being imaged.
Presently, the most sensitive method for imaging myocardial perfusion is through the use of coated microbubble contrast agents. There is a significant difference between the magnitude of an echo produced by a microbubble during destruction, as compared to an echo produced after the gas has dissipated. Accordingly, a region into which blood is perfusing can be recognized by evaluating two imaging frames in rapid succession, where a contrast agent is destroyed in the first frame, and then found to be absent from the region during the second frame.
U.S. Pat. No. 5,456,257, to Johnson et al., entitled xe2x80x9cUltrasonic Detection Of Contrast Agentsxe2x80x9d, (hereinafter xe2x80x9cthe Johnson et al. patentxe2x80x9d) describes a technique for detecting microbubbles through phase insensitive detection of microbubble destruction and differentiation of the detected signals on a spatial basis. A first ultrasonic pulse destroys the microbubbles in a tissue region and these destruction events are received and envelope detected. A second pulse is transmitted to the same locations, and the. returning echoes, ideally, show an absence of microbubbles at the locations where the microbubbles were destroyed. The second set of echoes is subtracted from the first set on a spatial basis, yielding difference signals of substantial magnitude at the locations where the microbubbles were destroyed.
The method of subtracting envelope-detected signals as described in the Johnson et al. patent suffers from poor sensitivity and susceptibility to clutter caused by echoes from structures other than an organ of interest. Unfavorably, this method also responds directly to a system gain change. Consequently, a user may not clearly determine whether an image represents blood perfusion, or lack thereof, or an artifact of the system gain.
Accordingly, there is a need for an improved method for detecting a relative level of a contrast agent in a region of a body.
There is also a need for such a method that further distinguishes between a flow of blood into the region and a perfusion of blood into the region.
Furthermore, there is a need for an ultrasound system that employs the""improved method for detecting a relative level of a contrast agent in a region of a body.
In accordance with a first embodiment of the invention, a method is provided for detecting a relative level of a contrast agent in a region of a body, where the contrast agent is susceptible to destruction by a destructive ultrasound signal. The method comprises the steps of: determining a signal level from a first ultrasound echo that is produced in the region during a first time, which occurs while the contrast agent is present in the region; determining a signal level of a second ultrasound echo that is produced in the region during a second time, which occurs after destruction of the contrast agent; and determining a ratio of the signal level from the first ultrasound echo and the signal level from the second ultrasound echo, where the ratio indicates a relative level of the contrast agent during the second time as compared to the first time.
In accordance with a second embodiment of the invention, a method is provided for detecting a relative level of a contrast agent in a region of a body, where the contrast agent is susceptible to destruction by a destructive ultrasound signal. The method comprises the steps of determining an average signal level from a plurality of ultrasound echoes that are produced in a plurality of spatial subsets of the region; determining a signal level from an ultrasound echo that is produced in at least one spatial subset of the region; and determining an arithmetic relationship between the average signal level and the signal level, where the arithmetic relationship indicates a relative level of the contrast agent in the at least one spatial subset as compared to the plurality of spatial subsets.
A system is also provided for performing each of the methods.